The present invention relates to a self leveling device, more particularly an automatic leveling device adapted to be moved vertically along a vertical wall, which is especially suitable for calibrating the capacity of a cylindrical tank by measuring outer radiuses thereof.
In industrial areas, there are many hugh cylindrical tanks which are generally intended to store liquid or gas. For the sake of accuracy in the buying and selling of the liquid or gas, it is necessary to know the exact volumetric capacity of each tank relative to the height to which the tank is filled so that the exact volumetric content of the tank in relation to the level to which the tank is filled will be known.
When the tank is built, the capacity of the tank theoretically may be calculated based on the construction drawings. However, in actuality, especially as time goes by, metal deformation takes place in the tank due, for example, to the pressure exerted on the tank walls by the tank contents, so that the tank capacity gradually changes. Therefore, it is desired periodically to check the tank capacity. This involves taking measurements and making mathematical calculations. It is necessary for this purpose to know the actual radiuses of the tank at various locations thereof.
If the tank is cylindrical, the tank is imaginarily cut into many horizontal sections, and the circumference is measured for each section. The radius of each section of the tank is calculated from the circumference, so that the capacity of the tank can be calculated.
In measuring the outer circumference of a cylindrical tank, there has commonly been used a method in which a metal measuring tape is positioned around the horizontal section. The metal tape can be situated horizontally at the bottom of the tank. However, it is almost impossible to situate the tape horizontally around the tank at high places from the ground, since at such places it is not possible for anyone to walk around the tank and adjust the tape. Therefore, the tape measuring method is less accurate than desired.
Another method, more recently developed, is called the "optical reference line method". In this method, a reference circumferential line is at first exactly horizontally provided near the bottom of the cylindrical tank, and the length of the reference line is exactly measured. The tank is imaginarily cut into many horizontal sections as in the tape measuring method, and the circumference of each section is measured with reference to the reference line of the tank by means of an optical device already known.
In particular, when the optical method is used, as shown in FIGS. 1-3, there are required a winch 10 situated on a tank T, a target device 11 with a scale 12 to be moved vertically along the vertical wall of the tank T by means of a line 13 attached to the winch 10, and an optical instrument 14 situated outside the tank T. The optical instrument 14 used herein comprises a sighting telescope 17 for reading a value on the scale 12, the instrument 14 being known in the measuring field and, therefore, not being described in detail in the present specification.
The target device 11 is, as stated above, at its upper end connected to the line 13 and a tail rod 15 is pivotally connected to its bottom end. A wheel 16 is rotationally connected to the tail rod 15 for continuous contact with the vertical wall of the tank T. The scale 12 is connected to the tail rod 15 perpendicular thereto at the position where the wheel 16 is connected to the rod 15. Therefore, the scale 12 indicates the distance from the vertical wall of the tank T when the wheel 16 contacts the wall.
In carrying out the optical method, a circumferential, horizontal reference line L.sub.1 is at first determined near the bottom of the tank T. The reference line L.sub.1 may be a metal tape as used in the tape measuring method, and the length of the reference line L.sub.1 is exactly measured. Other imaginary, circumferential horizontal lines, for example, L.sub.2 -L.sub.7 are provided on the tank T, the lines L.sub.2 -L.sub.7 being equally spaced. Further, the circumference of the tank T is divided into, for example, eight imaginary, vertical lines A.sub.1 -A.sub.8.
The winch 10 is located on the tank T so that the line 13 is situated slightly radially outwardly (relative to the axis of the tank T) spaced from one of the imaginary vertical lines, for example A.sub.1. In this position, the winch 10 is operated for moving the target device 11 vertically. First, the scale 12 is located at the reference line L.sub.1 and is moved upwardly to be located, in turn, at L.sub.2 -L.sub.7. When the optical method is performed, the optical instrument 14 is located outside the tank T with the line of sight of the telescope 17 preferably approximately on a tangent to the tank T at points on the vertical line A.sub.1. The telescope 17 of the instrument 14 is pivotable about both horizontal and vertical axes so it can be aimed at the scale 12 of the target device 11 for reading the scale 12. The optical instrument 14 is levelled (by known conventional means) so that the vertical pivot axis of the telescope 17 is on a line corresponding to a plumb line. With the scale first located at the level of the reference line L.sub.1 the telescope 17 is aimed at an arbitrary point (preferably approximately midway) on the scale 12 and the numerical value at that point is recorded. The telescope 17 is locked to prevent further pivoting about the horizontal axis but is still free to pivot about the vertical axis.
When the target device 11 is then moved and the scale 12 is, in turn, located at the level of each of the lines L.sub.2 -L.sub.7, respective numerical values on the scale 12 are determined and recorded by aiming the telescope 17 at the scale 12 by pivoting the telescope 17 upwardly about the horizontal pivotal axis. Differences in the numerical values A.sub.1 L.sub.1 . . . A.sub.1 L.sub.7 respective are due to irregularities in the configuration, i.e. deformation of the tank T.
After the numerical values for the vertical line A.sub.1 are obtained, the winch 10 is moved in order to superimpose the line 13 over the vertical line A.sub.2. Also, the optical instrument 14 is moved, and re-levelled, so that the line of sight of the telescope 17 is approximately tangential to points on the line A.sub.2.
Then, numerical values on the scale 12 for locations A.sub.2 L.sub.1 . . . A.sub.2 L.sub.7 are obtained. In this manner, a numerical value for each of the AL coordinates is determined.
After all the numerical values are obtained, the average numerical value for each of lines L.sub.1 -L.sub.7 is calculated, viz., by adding the values for each location A.sub.1 L.sub.1, A.sub.2 L.sub.1, . . . and A.sub.8 L.sub.1, and then dividing by 8 (number of vertical lines) and doing likewise for each other level. The average numerical values l.sub.1 . . . l.sub.7 relate to radius of the tank T at each corresponding level. Since the actual radius R.sub.1 corresponding to L.sub.1 is calculated based on actual measurement of the circumference L.sub.1, actual radiuses R.sub.2 . . . R.sub.7 corresponding to L.sub.2 . . . L.sub.7 are indicated as follows: EQU R.sub.2 =R.sub.1 +(l.sub.2 -l.sub.1) EQU R.sub.3 =R.sub.1 +(l.sub.3 -l.sub.1) EQU R.sub.4 =R.sub.1 +(l.sub.4 -l.sub.1) EQU R.sub.5 =R.sub.1 +(l.sub.5 -l.sub.1) EQU R.sub.6 =R.sub.1 +(l.sub.6 -l.sub.1) EQU R.sub.7 =R.sub.1 +(l.sub.7 -l.sub.1).
The capacity of the tank T is calculated based on R.sub.1 . . . R.sub.7 considering thickness of the tank wall and other factors relating to the tank T. Such details are well known to persons working in this field and are described, for example, API Standard 2550 (ASTM Designation D 1220-65).
The self leveling device of the present invention is used instead of the target device 11 in carrying out the above optical reference line method. The substitutions of the self leveling device of the present invention for prior art target device 11 results in certain advantages. For example, in the conventional target device, when the vertical wall of the tank T is inclined, although the wheel 16 is always in contact with the inclined surface, the scale 12 is not oriented horizontally, because the scale 12 is oriented perpendicular to the wall of the tank T. The scale 12 inclines in accordance with the inclination of the wall of the tank T. Accordingly, the numerical value on the scale 12 obtained by means of the optical device 14 departs from the true value needed for a precise calculation of the tank capacity. Further, if it is windy, the target device 11 moves or swings. Consequently, the optical method cannot be carried out.
Accordingly, an object of the invention is to provide a self leveling device adapted to be used in the optical reference line method for calibrating a tank capacity, in which the outer configuration of the tank can be precisely measured.
Another object of the invention is to provide a self leveling device as stated above, which can be properly positioned on the wall of the tank regardless of wind conditions.
A further object of the invention is to provide a self leveling device as stated above, which can be readily manufactured.
A still further object of the invention is to provide a self leveling device as stated above, which can be manipulated easily without deviation.
Further objects and advantages of the invention will be apparent from the following description of the invention.